1. Field of the Invention
The present invention is directed to digital projection systems, and more particularly to a high efficiency dynamic color control management system.
2. Description of the Related Art
There is growing interest in high-quality digital projection systems that display images that can match or surpass the quality of film, especially in large venues. One solution utilizes a Digital Light Processor (DLP) as the image forming device.
One well known form of Digital Light Processing (DLP®) projection, originally developed by Texas Instruments, is based on the use of a source lamp, an illumination system, and a color splitting-recombining light engine. The optical function of the light engine is to split uniform illumination light into Red/Green/Blue (RGB) channels, direct the three channels onto respective spatial light modulators, such as imaging devices or optical panels such as an LCD (Liquid Crystal Display) or DMD (Digital Micromirror Device), and then re-combine all three channels into a single illumination light beam that is projected onto a screen via a projection lens.
The DMD is a MEMS (microelectromechanical) device consisting of microscopic mirrors that modulate light by independently flipping each mirror through a +−12 degree angle, between an “on” state for reflecting the source light to be viewed on screen and an “off” state for diverting the light to a light dump. Intermediate light intensities on screen are generated by toggling the DMDs according to a pulse width modulation (PWM) method, at frequencies above human perception in order to achieve time-averaged values. In this respect, the exact grey level produced depends on the pulse-width-modulated (PWM) duty cycle.
Conventional high luminance projectors currently use discharge lamps as light sources (e.g. Xenon arc lamps), although projectors that use solid state light sources, such as LEDs and lasers, are beginning to emerge on the digital projector market. Projectors that use solid state light sources offer advantages over discharge lamps in terms of longer lifetime and increased reliability. Furthermore solid state light sources can be characterized by a widely increased color gamut, and greater spectral purity.
Whether a projector uses a conventional arc lamp or newer laser or LED illumination, key design objectives include dynamic range, power conservation and lifetime of the illumination source.
A system commonly referred to as a “dynamic iris” has been used for many years in lamp-based projection systems to increase dynamic range from maximum peak light output levels to the darkest scene (i.e. “black level”). The dynamic iris is positioned between the lamp and the DMD. Many times per second, the projector evaluates the overall brightness of the image being projected and then opens or closes the iris to allow more or less light through. The on/off contrast rating is based on the whitest white with the iris open, and the blackest black when the iris is closed. However, not all of the light generated by the illumination source is actually used. For example, light may be absorbed by the iris (i.e. does not reach the DMD) or is “dumped” by the DMD. Consequently, dynamic iris systems do not result in a reduction in lamp power usage. Since lifetime of the illumination source is inversely proportional to the power drive levels of the light source, there is therefore no increase in lamp lifetime.
The inventors have discovered that the average brightness levels or APL (average picture level) is often less than 50% for most video and movie content. Peak pixel levels can also be much lower on a frame-by-frame and color-by-color basis. Many scenes are very dark in nature with no pixels requiring full brightness levels and therefore also would not require full power illumination for correct display levels. Also, the light output of a discharge lamp cannot be quickly or reliably modulated. Therefore, for the bulk of content, light is generated by the lamp that is not being used to project an image. This has many costs, notably in terms of system efficiency (wasted energy to generate unused light), thermal management (the unused light is lost as heat within the system), and source lifetime (lowering the power can increase the time to failure of a light source).
The inventors have discovered that if the input power level to the projector can be reduced, while maintaining output power for allowing the brightest pixels to be displayed at peak colour or luminance, then the light source lifetime can be significantly increased, and the black levels of the darkest scenes decreased thereby increasing the observable contrast ratio.